Photon migration has been investigated experimentally in vitro on human breast tissues, bovine liver, tissue phantoms, and theoretically by Monte Carlo simulations and diffusion theory. The spatial intensity profiles have been measured at the output surface of a sample illuminated by a collimated beam. Experimental results have then been compared with simulations that assume the sample to be homogeneous. Measurements on phantoms, i.e., fat emulsion and microspheres suspension, and on liver are in good agreement with theory. On the other hand, the width of the intensity profiles measured on breast tissues (adipose and fibrous) are systematically larger than those measured on phantoms or calculated by simulations. The structure of these samples, not considered in simulations and not present in phantoms, explains these differences.